A conventional power sliding apparatus for a seat is disclosed in a Japanese Patent Laid-Open publication published as No. 10(1998)-968. The disclosed power sliding apparatus includes an upper rail supporting a seat mounted thereon and a lower rail fixed to a vehicle floor and arranged to be slidable relative to the upper rail. The upper rail is equipped with a screw rotatably driven by a driving unit and the lower rail is provided with a rack having plural teeth defined in a vehicle longitudinal direction. When the screw arranged to be engaged with the rack of the lower rail is rotatably driven by the driving unit, the upper rail can be slidably moved relative to the lower rail in response to the rotation of the screw. Therefore, the power sliding apparatus can effectively adjust the seat position in the longitudinal direction.
When this type of power sliding apparatus is applied to adjust a vehicle seat position, the power sliding apparatus is required to be sufficiently solid and durable against excessive load which can be applied to the vehicle seat in the longitudinal direction due to a vehicle crash and so on. Otherwise, the seat position may not be able to be effectively maintained at an intended position.
According to the conventional type of power sliding apparatus, the screw and the rack are required to be made of metal so as to ensure the robustness of the power sliding apparatus and in order to achieve a downsized power sliding apparatus for an actual use in the vehicle. However, when the screw and the rack are in contact and engaged with each other with no gap therebetween, deterioration of durability thereof may be resulted from large friction generated therebetween. To overcome the problem, the screw and the rack can be arranged possessing a gap therebetween on a designing stage in order to ensure the durability. However, another problem may occur, in which looseness in the vehicle longitudinal direction between the upper rail and the lower rail may be caused and squeaks and rattles may occur when the power sliding apparatus is operated for positioning the vehicle seat.
The present invention therefore seeks to provide an improved power sliding apparatus capable of reducing the looseness between the upper rail and the lower rail and undesired noise. Further, the improved power sliding apparatus is able to ensure requisite solidness against impact load.